The advantages of water as a diluent for surface coatings is well known, as for example, such a diluent is both nonflamable and relatively inexpensive.
The use of water does require; however, an adaptation of the customary binders. Widely used surface-coating compositions are those containing a binder that is made soluble in water. Carboxyl groups are usually incorporated into a binder to an acid number higher than 30, which are then completely or partly neutralized with a base. Since many binders are polymeric substances, the high viscosity of these solutions sometimes presents difficulties, especially when a high concentration is required. Another difficulty is the long drying time that is often required before the surface coating can be enamelled. If water is still present in the surface coating when the enamelling step is performed, blistering of the surface coating may result.
Aqueous dispersions in which the binder is dispersed in water instead of dissolved, have as advantages that higher binder concentrations can be used without viscosity problems, and that it is easier to evaporate the water from the surface coating than from a solution; a drawback is usually that surface-active substances or dispersants have to be used for making the dispersion, and for giving it a sufficient stability. Such surface-active substances or dispersants generally have an unfavorable effect on the properties of the hardened surface coating; in particular, they increase the water sensitivity.
A process has now been found for preparing resins that are dispersible in water without using surface-active substances or dispersants.